For determining the effect of environmental pollutants on fish bone metabolism, we have developed an in vitro bioassay system using teleost scales that has osteoclasts, osteoblasts and bone matrix as markers: alkaline phosphatase (ALP) for osteoblasts and tartrate-resistant acid phosphatase (TRAP) for osteoclasts. Using this bioassay, the influence of gadolinium (Gd) on osteoclasts and osteoblasts of goldfish scales was examined in the present study. Gd sensitively inhibited TRAP activity. Even Gd at 10-13 M suppressed TRAP activity at 6 hours of incubation. At 18 hours of incubation, this inhibition occurred only at 10 7 and 10 6 M. After 36 hours of incubation, Gd did not influence TRAP activity. In osteoblasts, ALP activity was also suppressed by Gd in the range of 10 10 to 10 6 M for 6 to 18 hours of incubation. At 36 and 64 hours of incubation, ALP activity was significantly suppressed by Gd (36 hours: 10 9 to 10 6 M; 64 hours: 10 7 and 10 6 M). At 96 hours of incubation, however, Gd did not influence ALP activity. This is the first report to indicate the toxicity of Gd on fish bone metabolism using TRAP and ALP enzyme activities. The toxicity of Gd to osteoblasts is comparable to that of tributyltin, an aquatic environmental pollutant used as a biocide in anti-fouling paint. Gd is used in Magnetic Resonance Imaging (MRI) for clinical diagnoses. To avoid the toxicity of Gd ions, chelated forms, known as Gd-based contrast agents, are used for MRI diagnosis. Without a specific recycling process, these compounds are quickly released by urinary excretion and released into environmental waters. Therefore, it is possible that anthropogenic Gd influences aquatic animals. Considering our present data together with that of anthropogenic Gd pollution, we should conduct a Gd risk assessment to protect the ecosystem in the aquatic environment.

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